1. Field of the Invention
This invention relates to an optical device which is applied to a recording and/or reproducing apparatus for recording and reproducing information signals for an optical recording medium such as optical discs, a method for producing the device, and a recording and/or reproducing apparatus employing the device.
2. Description of Related Art
There has been proposed an optical disc apparatus in which an optical system is arranged on the side recording layer of an optical disc to achieve a high numerical aperture (NA) and hence the high density recording. This optical disc apparatus uses, as an objective lens used in an optical pickup, an optical device having two lenses, such as is shown in Japanese Laying-Open Patent Publication H-10-123410, as light converging means.
Of the two lenses of the optical device, shown in this Publication, the one lying towards the optical disc is a so-called hemi-spherical lens. This lens towards the optical disc and the opposite side lens are hereinafter referred to as a forward lens and a rear lens, respectively. In this optical device, the numerical aperture (NA) is determined by a diaphragm which is a light transmitting aperture provided ahead of an incidence point of a light beam radiated from a light source on the optical device. In this optical device, the light beam for signal recording, incident on the optical disc, and the light beam for reproducing the information signals recorded on the optical disc, are converged by the forward and rear lenses to fall on the optical recording layer of the optical disc.
Meanwhile, in an optical device, used in a conventional optical disc apparatus, the light beam converged by the forward and rear lenses is illuminated on the optical recording layer of the optical disc. The optical device, made up of the forward and rear lenses, is designed to suppress an eccentricity from the center axis of the light beam due to tilt of the optical axis or to assembling errors to a smallest possible value.
In an optical disc system, a high data transfer rate, a higher band for not only the focusing servo and tracking servo but also an actuator, and reduction in weight of the actuator, are being sought. For reducing the weight of the actuator, it is indispensable to reduce the size and the weight of the optical device loaded on the actuator, such that a higher assembling accuracy is required of the optical device. If the optical device, thus reduced in size and weight, is constructed so that the numerical aperture (NA) is determined by the diaphragm, the slightest deviation from the center of the optical axis of the light beam illuminated on the optical disc leads to marked variations of the diameter of the light beam illuminated on the optical recording layer.
In realizing the high density recording of the optical disc, reduction in the spot size of the light beam illuminated on the optical recording layer of the optical disc is a requirement. For reducing the spot size of the light beam, it becomes necessary to increase the numerical aperture (NA) of the objective lens used for converging the light beam.
Meanwhile, if desired to realize a large numerical aperture (NA) by a single lens, a large refracting power is required. If the refracting power is increased, the radius of curvature of the objective lens becomes smaller, with the result that the position matching tolerance of the refractive surfaces becomes narrow. Consequently, a limit on the order of 0.6 is set for the numerical aperture (NA) for a single lens.
With the lens set, made up of two lenses, that is the forward and rear lenses, as described in the aforementioned Publication, it is possible to increase the numerical aperture (NA). With this lens set, it is necessary to provide for a constant separation between first and second lenses and for accurate orientation of the second lens with respect to the first lens.
The separation between the first and second lenses and the setting of the orientation of the second lens with respect to the first lens are based on the outer profile of the respective lenses. With the first and second lenses, produced by injection molding employing metal molds, the outer profile of the lens needs to be trimmed to high precision. With the molding by the metal molds, it is only possible to mold the lens to a limited degree of precision, whereas, with the position matching based on the outer profile, it is difficult to achieve high precision position matching. If the first and second lenses, assembled together, are not positioned accurately relative to each other, the second lens may be placed at a separation different from the design separation, tilted or offset with respect to the first lens. If such variation in the separation, tilt or offset is produced, there is generated an aberration exceeding an allowable range prescribed for a single lens, for example, 0.4 rms.
Among the lenses used for an optical pickup, there are a plastic lens produced on injection molding of a synthetic resin, a glass lens produced on glass molding, and a glass lens formed on polishing. With these lenses, it is unexceptionally difficult to reduce the radius of curvature to render it difficult to produce a small diameter optical lens. In order to overcome these inconveniences, such a lens having a continuously curved surface, formed by applying the manufacturing method for a Fresnel lens, has been proposed. However, it is still difficult with this proposed method to produce a high precision lens.